Quick Notes Enthalpy Changes

  • During chemical reactions energy is exchanged between reacting substances and their surroundings.
  • Exothermic reactions – products are lower in energy than the reactants and the temperature of the surroundings increases.
  • Endothermic reactions – products are higher in energy than the reactants and the temperature of the surroundings decreases.
  • Enthalpy is a measure of heat energy with the units kJ mol-1.
    • The actual amount of energy in a compound or atom is not easily measured in chemistry, so it is the change in heat energy during a reaction (enthalpy change, ΔH) that is measured.
    • Exothermic reactions have negative enthalpy changes (-ΔH).
    • Endothermic reactions have positive enthalpy changes (+ΔH).
  • Standard enthalpy change refers to the thermal energy change that occurs during a reaction, where 1 mole of product is formed in standard conditions (101 kPa and 298K).

Full Notes Enthalpy Changes

When chemical reactions happen, the products produced (nearly always!) have different energies to the reactants. This means energy has either been gained or lost by the reactants.

Energy cannot be created or destroyed, only transferred from one ‘store’ to another. During chemical reactions, energy is transferred between the reactants and the surroundings. The surroundings just refer to anything and everything around the reaction that is not actually reacting (for example, a gaseous atmosphere around the reaction).

If the surroundings gain energy, the temperature of the surroundings increases. If the surroundings lose energy, the temperature of the surroundings decreases.

If energy is released in a reaction process, then the reaction is described as being exothermic. Exothermic reactions have negative energy changes as energy is transferred from the reactants to the surroundings.

If energy is absorbed in a reaction process, then the reaction is described as being endothermic. Endothermic reactions have positive energy changes.

 

energy profile diagram reaction A-level chemsitry exothermic endothermic positive negative change enthalpy

 

Energy is transferred in the form of heat energy. In an exothermic reaction, heat energy is released from the reactants to the surroundings – the temperature of a reaction’s surroundings increases. In an endothermic reaction, heat energy is taken from the surroundings and absorbed by the reactants – the temperature of a reaction’s surroundings decreases.

The first law of thermodynamics tells us that the total energy in a closed system remains constant. A closed system is one where no energy can get in or out (imagine a perfectly sealed container surrounded by a perfect vacuum).

In chemistry, we do not measure the actual amount of energy in a particular substance. Instead, we measure the energy change that occurs when substances react and form a new species. This energy change is called the enthalpy change(ΔH). This enthalpy change enables us to compare the energy of the reactants to the product(s).

Enthalpy change (ΔH) is measured as a change in temperature (heat energy) at a constant pressure.

If we want to measure and compare enthalpy changes for different processes, we have to make sure they are being measured in the same conditions.

We call these conditions ‘standard’. Standard enthalpy changes are enthalpy changes that have been measured at a particular pressure and stated temperature. The symbol for standard enthalpy change is ΔH.

Standard conditions are 101KPa and 298K (25oC). This is NOT the same as room temperature and pressure, check your exam board and make sure you know the values required.

What is STP?

This is where things get even more confusing, STP refers to Standard Temperature Pressure and the values used are 273.15K and 100 KPa. Very simply, STP is used for ideal gas calculations (see Moles – Gases), whereas standard conditions are used for ‘everyday’ enthalpy changes and experiments at A-level.

Standard state refers to the state of a substance under standard conditions.

We also need to know how much of our substance is reacting! The standard amount of substance used is always 1 mole.

For example:

The standard enthalpy of combustion (ΔHc) is the change in enthalpy that occurs when 1 mole of a substance undergoes complete combustion under standard conditions.

The standard enthalpy of formation (ΔHf) is the change in enthalpy that occurs when 1 mole of a substance is formed from its constituent elements in their standard states.